Over Pressurized Coolant System???
 

Been chasing a problem for a while now and time to seek advise from the experts... '03 X Type has been losing about 1/3gal. coolant in 120mi drive (daily). I have had cooling system apart, inspected entire system for visible leaks, flushed cooling system, etc... Replaced reservoir thinking there might be a hair line crack contributing to coolant collecting on top of tranny. New reservoir/cap did not fix, still losing about the same. Put leak detection dye in coolant and was able to determine that its coming from reservoir cap, I'm guessing over pressurized system??? If so, may be linked to an issue a few months ago when the upper radiator hose popped off the rigid pipe directly below the oil fill. Reinstalled hose, filled with coolant and went on my way. No noticeable coolant loss for several weeks following that occurrence.

What could cause over pressurization of the cooling system? Head gasket possibly, but no coolant in oil or oil in coolant and no noticeable white smoke in exhaust. I've tried a few different reservoir caps, to no avail.

Its getting costly constantly filling the reservoir (daily)and unnerving waiting for a larger issue to surface...

Any thoughts?
Thx

Are you overfilling not leaving room for coolant expansion when it gets hot?

Get a coolant system pressure tester (often cheap on the used market) and first do a cold-bleed test (pump it to operating pressure, then watch a couple of minutes for a drop in pressure, indicating a leak) then a hot test (see what you have when the engine is running at operating temperature).

If the needle vibrates or twitches during the hot test, that indicates a leak in one or more of the cylinders (most likely head gasket).

Have you checked this?

https://www.jaguarforums.com/forum/x...engine-217754/